Solder joints are widely used to join two metallic materials, providing a bond having electrical conductivity, low thermal impedance, and mechanical durability. Depending on the particular application in which they are being utilized, solder joints may be relied upon for their ability to accommodate thermal expansion stresses, to form a mechanically sound joint that is stable across a range of temperatures, to resist moisture, and to provide low thermal impedance, for example. In the semiconductor device applications, a solder joint can be used to join an active device, such as a microprocessor semiconductor die, to a heat spreader. Used in this way, low thermal impedance and uniform heat dissipation are keys to satisfactory solder joint performance.
A conventional method for forming a solder joint between metal surfaces typically include depositing flux on the metal surfaces, placing a solder material between the surfaces, and then heating the solder material to form the solder joint. In the conventional method, the flux, which outgases as the solder is heated, can disperse into the solder material and form “voids,” which can, for example, reduce the mechanical strength of the solder joint. More problematic for semiconductor device applications is that a void in a solder joint can act as effective local insulator, resulting in sharply increased thermal impedance around the void. The presence of insulating voids in a solder joint used to attach an active semiconductor device to a heat spreader, for example, may result in overheating, damage, and ultimately failure of the device. Thus, voids can undesirably reduce the effectiveness of solder joints.